This invention pertains generally to transport apparatus and more particularly to transport apparatus in which a length of pliant material is fed between supply and take-up rolls in edge driven relationship with a drive capstan.
In transports of the type described in U.S. Pat. Nos. 3,370,803 and 3,370,804, for example, a pliant material such as magnetic recording tape is wrapped to form supply and take-up rolls, the outer edges of which are engaged directly by a drive capstan to feed the tape between the rolls. In order to maintain control of the tape in the region between the rolls, the take-up roll must be pressed against the capstan with a greater force than the supply roll. This force differential causes the tape to pass through the interface between the take-up roll and capstan at a greater velocity than that with which it passes through the supply roll/capstan interface, thereby maintaining a slight tension in the tape and preventing the formation of undesired tape loops between the rolls.
To prevent formation of such loops, the proper forces must be established immediately when tape movement begins, and the force magnitudes must also be reversed immediately each time that the direction of tape movement is reversed. This is particularly important when the direction of tape movement is reversed repeatedly, as, for example, in the recorders utilized in digital operations, because, under these circumstances, tape control is more difficult to maintain than in the case of analog recordings where the tape is fed continuously in one direction.
Force systems heretofore provided are of two basic types--dynamic and static. In dynamic systems there is a source of power, such as a motor or other rotating component, which is utilized through various coupling means to urge the take-up roll against the capstan with a force greater than that urging the supply roll against the capstan. The '803 and '804 patents noted above disclose such systems. In the former, a string wrapped about the capstan shaft transmits power to the appropriate (take-up) carriage, and in the latter, strings wrapped bout the roll spindles draw power from these spindles as they rotate, to respectively urge the take-up roll toward the capstan and the supply away from the capstan. Also, in U.S. Pat. No. 3,408,016, it is shown how electric motors may be utilized to urge the roll carriages toward the capstan with appropriate forces.
These dynamic force systems generally meet the requirements for immediately urging the carriages against the capstan with appropriate force regardless of direction of tape motion or the rapidity with which this direction is changed. They are, however, not suitable for all requirements, in that they may be too expensive, or the power source may be inconvenient or even impossible to couple to the carriages, or, in some cases, may be insufficient for the purpose.
To overcome these objections to dynamic force systems, various static force means have been provided, as for example in U.S. Pat. Nos. 3,370,804 and 3,960,342. In such systems, no source of power is utilized to urge the take-up carriage toward the capstan, but rather, simple static forces (e.g., friction or spring) are used to yieldably resist the movement of the take-up roll away from the capstan as roll diameter increases when tape is wound thereon. In this manner a desired take-up force--even a very high one--is easily obtained, by merely increasing or decreasing friction or the strength of a spring, in order to increase or decrease the resistance to the movement of the take-up carriage away from the capstan.
Static forces in general have a serious deficiency, however, inasmuch as mere reversal of the direction of tape movement will not immediately and automatically reverse the magnitudes of the static forces acting to press the tape rolls against the capstan.
In the static friction force systems previously disclosed, such as '804, for instance, sliding friction between the carriages and the transport base is constantly present and resists movement of the carriages in either direction (toward or away from the capstan). A spring common to both carriages and of sufficient strength to overcome this frictional force is coupled between the carriages and acts to draw them toward the capstan so that the tape rolls are maintained in contact with the capstan at all times. As a result, the movement of the carriage which has been operating in the take-up mode (e.g., forced to move away from the capstan as the roll diameter increased) has been resisted by the combined forces of friction and that of the common force spring, and penetration (indentation) of the capstan tire by the take-up roll is therefore relatively great. On the other hand, penetration of the supply roll into the capstan tire is relatively little, inasmuch as it has been urged toward the capstan as its diameter decreased, by only the force of the common spring minus the resistive force of friction. In short, the required force differential is brought about only because the roll diameters change during winding and unwinding of tape, forcing the carriages to move against greater static resistive force in one direction than the other, as a result of which one roll presses against the capstan with a greater force than does the other.
The problem with such a friction force system derives principally from the fact that as long as the direction of tape movement remains unchanged, the prevailing forces will also remain unchanged, even if tape movement ceases altogether or proceeds intermittently. These circumstances would be desirable for completely unidirectional operation, but act to defeat rapid force reversals during bi-directional operation. If the forces are established for a particular direction of tape movement and remain unchanged when the tape stops in the normal course of a reversal, it will be evident that when the tape begins to move in the opposite direction, it will do so with the forces exactly opposite to what they should be: the new take-up roll will be pressing into the capstan with relatively little force, and the new supply roll will be pressing against the capstan with relatively high force. This situation will continue to a diminishing extent until enough tape has been removed from the new supply roll and wound onto the new take-up roll to force the new take-up carriage to start moving against the frictional resistance. Until this force reversal is literally complete, tape control will be compromised, and failure is likely to occur.
Static spring force systems such as described in '342 present somewhat different problems, particularly in the practical sense. In the simplest such system, a spring is attached to each of the carriages and is alternately tensioned so that the carriage acting in the take-up mode (moving away from the capstan) will always be urged toward the capstan with greater force than is the other (supply) carriage. In order to prevent the occurrence of force changes as carriage positions change, low rate springs are generally used, and because of this considereable movement of the spring anchor points will be required to tension or untension the springs. With such systems, there is no automatic reversal of the spring forces upon reversal of the direction tape movement, and factors such as the strength of the springs and the amount of movement required to tension or untension them have made it impractical to utilize electromechanical devices for reversing the forces of the springs.